1. Field of the Invention
The present invention relates to a parallel mechanism, and in particular, to a parallel mechanism having a turning shaft.
2. Description of Related Art
A conventionally known parallel mechanism includes a base portion that is a support base and a bracket with an end effecter attached thereto are coupled together in parallel through a plurality of links. In the parallel mechanism, for example, actuators such as electric motors are arranged in parallel, and the plurality of links (arms) coupled to each of the electric motors eventually operate the single end effecter.
Compared to a joint mechanism such as a serial mechanism, the parallel mechanism configured as described above eliminates the need to provide an electric motor or the like for each joint and to swing around the electric motor or the like provided in the joint. Thus, the joint mechanism can be made lightweight. Furthermore, in the parallel mechanism, the forces of all the electric motors or the like concentrate at one point, allowing a high output to be provided.
Moreover, the parallel mechanism has a triangular pyramidal structure and thus offers a high rigidity. Thus, because of the above-described characteristics, that is, the light weight, high output, and high rigidity, the parallel mechanism allows the end effecter to be operated at a very high speed. Thus, the parallel mechanism is used in, for example, applications requiring a quick repetition of an operation of moving the parallel mechanism to a conveyance target, allowing the end effecter to grip the conveyance target, and then allowing the end effecter to hold and convey the conveyance target to a predetermined position. However, when the parallel mechanism is reciprocated at high speed, the end effecter may be vibrated. Then, upon gripping the conveyance target or releasing the conveyance target at a predetermined position, the end effecter may be disadvantageously inaccurately positioned. Thus, there has been a demand for allowing the end effecter to be accurately positioned upon reaching the target position, without an increase in time required for the reciprocation (that is, a demand for reducing the misalignment of the end effecter when the end effecter grips or releases the conveyance target).
Furthermore, a parallel mechanism according to a certain conventional technique includes a nested link mechanism (turning shaft) that couples a gripper (end effecter) rotatably supported on a carrier (bracket) to a servo motor fixed to a base portion. The opposite ends of the link mechanism are connected through Cardan joints (universal joints). Thus, even when the link mechanism is tilted as the end effecter moves, the rotational driving force of an electric motor can be transmitted to the end effecter, which can thus be rotated.
The parallel mechanism configured as described above is suitably used for applications such as a palletizing operation which require a quick repetition of an operation of, for example, moving the parallel mechanism to a conveyance target, such as a packaged food or a solar cell, allowing the end effecter to grip the conveyance target, and then allowing the end effecter to hold and convey the conveyance target to a predetermined position. Here, the palletizing operation may require an operation of, for example, arranging rectangular work pieces transferred in different orientations in the appropriate orientation and then placing the arranged rectangular work pieces in a case partitioned into blocks. In this case, during the conveyance of the gripped work piece, the turning shaft and the end effecter are rotated to place the rectangular work piece in the case with the orientation (rotation angle position) of the work piece aligned with one of the blocks in the case.
Here, as described above, the turning shaft provided in the parallel mechanism is composed of mechanical elements connected together in series and including a Cardan joint (universal joint), nested shafts configured to be slidable and non-rotatable with respect to each other, and a Cardan joint; the Cardan joint, the nested shafts, and the Cardan joint are arranged in this order. When rotationally driven, the turning shaft is twisted as a result of the inertia moment and loosening of each of the mechanical elements in connection with the rotating force of the electric motor. Since the mechanical elements of the turning shaft are connected together in series as described above, the end effecter attached to the tip of the turning shaft is particularly significantly twisted. Furthermore, transmission of the rotating force from the electric motor to the end effecter suffers a time delay. Thus, after the rotation of the electric motor is stopped, the end effecter may overshoot a target rotation angle and vibrate. In particular, the twisting and vibration caused by the loosening do not substantially attenuate and thus require a long time until the twisting and vibration converge. Furthermore, in spite of a constant motor rotation number, due to a character of the universal joints, the driven shaft between the joints is subjected to variation in angular speed, angular acceleration, and torque. Thus, where the driven shaft between the joints offers a large inertia moment, the vibration of the end effecter may be affected. As a result, when the end effecter grips or places the work piece at a predetermined position (when the end effecter reaches a target position), the actual rotation angle position may deviate from the target position.